Is this one of those tester thingee's, which is like a small Screwdriver with a neon Lamp inside a plastic casing; and to perform testing, the blade is placed on an ungrounded "Source" ("Hot" wire's lead), then the "Tester" touches a Silver colored contact - which "should" make the Lamp light up?

If yes, I really have no definite answer - as I have only seen persons using these things in the past - and even then it was very infrequently seen!Never used them myself.

One guess would be the testing person is completing the circuit to either the Floor (Earth Ground) or to a Grounded/Bonded piece of metallic equipment.Likely the connection is something direct - and the level of current flowing through the complete circuit (the Lamp and the "Testing Person") would be very low - in the range of 100 MicroAmps (0.1 milliAmps, or 0.0001 Amps) maximum.

Other option would be something being Capacitively Coupled to the AC supply through the Tester and Testee (pun, definitely intended!).

Scott hit it with the "testee" completing the circuit. Inside the screwdriver handle is a small neon lamp and a series resistor. When you hold the clip and touch a hot terminal you are actually completing the path for the current through your body.

You don't feel anything, because the current involved is very tiny. The series resistor by itself would limit current to a milliamp or so, but the actual current drawn by the neon is in the order of microamps.

It's because the neon is an inherently high-impedance device that it lights even though the resistance to ground via your body might be very high.

You can try an experiment:

Stand on the floor wearing dry slippers or rubber-soled shoes and touch the screwdriver to a hot terminal. It will light, but probably rather dimly. Now take off your shoes (and socks if you like) and repeat the test. You'll find that as the resistance to ground via your feet is now lower that the neon will be considerably brighter.

But I still don't understand.Which is the main cause to light up that small neon bulb?

The circuit consists of AC hot wire,tester(neon screwdriver),testee(person)and ground? OR the "Capacity Coupled to the AC supply through the Tester and Testee (pun, definitely intended!)"?I was confused!Because I have done the experiment that suggested by pauluk.But this experiment can not tell which one is the main cause!!!

Further more, does the capacity effect mentioned by Scott35 realy exists?As you know,hot wire,tester and testee can NOT form a circuit without ground!!!So,I am confused!!

If I could made this question clear,I would quite understand the theory of AC power system.So this question is very important to me,Can anybody help me??!!!Thank you again!!

I will try to explain this, and give some concise answers, but at the same time I might wind up leaving you with no answers and more questions!(just joking!)

If you performed the experiment which Paul described, then the Tester uses You as two things:

[*] Part of the Circuitry,

[*] Part of the overall Loop Resistance.

The Current flows through your body to/from the surrounding Floor area, and eventually connects to the AC System via a "Ground Path" somewhere, somehow.This could be to a Concrete Floor - connecting to the GES and eventually to the AC System, or could be through some Semi-Conductive Material - to a Bonded Piece of Equipment, then back to the main Bonding Jumper and eventually connects to the AC System.

As Paul's description was pointing towards, if you remove your Shoes and Socks, the Neon Lamp may glow brighter (and you may also begin to feel the Current flowing!) - this would suggest the Loop ("Circuit) is formed by using the Floor as temporary connection to the AC System.

This Loop's Conduction may be direct ("Normal" Closed Loop), or may be a Capacitive Coupled situation, which results in Current flowing through your Foot to/from the Floor, via the "Capacitor" created by your Shoe. (very extremely brief description!)

I need to get hold of one of these testers, in order to perform some experiments.

I can understand the capacitance effect created by shoe in the circuit consist of hot wire,tester,testee,ground,transformer and back to the hot wire.But what I am thinking about is this question: Can hot wire,tester and person form a capacitance effect without the ground???That is to say the bulb can light without a circuit!!!!My theory is that hot wire contains a variable sinusoidal voltage(+170,-170)to ground.While the voltage is changing,the electrons is being drawn in and out the testee body,so there is a variable current flow though the bulb between the hot wire and testee,and the bulb is light,furthermore the tester contains a high value resistance(1M),it just like dielectric in a capacitance!!!In this theory,even the person is 100% insulated from the surrounding,the tester can also check out the hot wire!!!!!!!!!But I don't know if my theory is right!

As I understand your question, you're asking whether the primary reason for the neon lighting is resistance or capacitive coupling.

In the sort of arrangement we're discussing there are bound to be elements of both. If you are standing in damp shoes or barefoot on a concrete floor, then the resistive angle is almost certainly responsible for almost the full amount of current which will flow.

As you increase the actual resistance though, any capacitance which exists will become a more significant part of the equation.

it might be educational to take a known high-value insulator such as a sheet of glass and try using a neon tester while standing on it. I've not tried this experiment, but I would think that there would be sufficient capacitance to get the neon to glow a little, unless the floor beneath the glass provided an exceptionally high resistance to ground.

I too have one of these mystery testers. It was given to me by an old friend a couple of years ago. Always wondered how it worked bu never really tried any practical tests with it. It must work through some kind of capacative coupling though, as it will light just touching the insulation of a live wire no direct elecrical contact. I'll have to play with it some more.

In the interests of ECN members, and of providing further evidence that we English are completely craz..... I mean eccentric , I've just carried out a few practical experiments.

Keep in mind that I was using a supply at 240V RMS to ground, and a neon tester designed for such which probably has a higher series resistance than the ones sold in the States.

First up, with the tester hardwired directly across 240V, I obtained a current of 180 microamps, so that's the maximum possible. The rest of the tests were carried out with my thumb on the button and a meter in series with the hot supply to the tip.

Standing barefoot on the dry lino in my kitchen gave a reading of around 60uA. Peeling the lino back and standing directly on the floorboards increased this to about 80uA (and these boards are on about 1-inch battens which in turn are resting on a solid concrete slab).

I don't happen to have any glass laying around, but I tried various combinations of carpet offcuts, paper lining, etc. to stand on and the lowest current obtainable was about 20uA, at which point the neon was obviously considerably dimmer than before.

That could still be due to resistance, so I decided to use air as an insulator by jumping up and down. I couldn't stay off the floor long enough to get any sort of reliable reading on the meter, but I can confirm that the neon dimmed but was still alight when my feet were off the floor by about a foot or so.

I think that proves that the human body can provide enough stray capacitance to the surroundings to keep the neon lit.

And NO, I don't have a photo of me barefoot in my kitchen hanging on to test leads while jumping up and down!

I got one of these many years ago when I would help out the residential part of the company. They seemed like the only way to test knob and tube or the old houses with switched neutrals back in the day. Any way I was also trying to figure out exactly what was going on so I was in the basement, touched a hot wire with the tester and it glowed dimly. I tried it again and touched a grounded pipe and it glowed very brightly. It's been quite a few years but I think it felt like a static shock on a cold dry day if you touch the grounded pipe during the test. My conclusion was that I was a parallel path to ground and haven't thought much about it since. I'm just thankful I gave up before I started jumping up and down barefoot in the kitchen and told the world.